The ribosomal RNA genes of Saccharomyces cerevisiae constitute a cluster of 140 tandemly arrayed 9Kb repeat units. The sequence of the rDNA repeat varies between strains of yeast, but within any one strain all the repeats appear to be identical. The process by which this sequence homogeneity is maintained is called rectification. Several mechanisms have been proposed for this processs: master-slave correction, democratic gene conversion, saltatory replication and unequal crossing over. I have used recombinant DNA techniques to construct strains of yeast with genetic markers inserted into the rDNA locus. These strains were used to demonstrate the occurrence of unequal crossing over. A preliminary analysis of deletions and duplications of the insertion showed that the rate of unequal crossing over is sufficient to account for rectification in yeast rDNA. In this proposal I describe the construction and use of new rDNA insertions designed for the study of recombination within the rDNA locus. Unequal crossing over will be studied in more detail with insertions that vary in size and structure. Preliminary evidence has been obtained for several other types of recombination in rDNA, all of which could contribute to rectification. Hybrid plasmids will be constructed for the detection of specific classes of events. These plasmids will also be used to study rDNA magnification and reduction which may occur by the same mechanisms as sequence rectification. Strains with two or more different insertions will be used for the selection of large rDNA deletions by simultaneous loss of both markers. These deletions will be examined for deletion size, effects on cell growth, and the rate at which the wild type copy number is restored. Computer simulations of sequence rectification and copy number variation will be used to assess the evolutionary significance of the various classes of recombinational events measured in the laboratory.